ci: bump github actions to versions that natively target Node 24

Covers the "Node.js 20 is deprecated" warning emitted by actions/checkout,
actions/setup-node, actions/cache, actions/upload-artifact and
actions/download-artifact on their older releases.

Every bumped line carries an inline `# kilocode_change` marker since these
workflow files are shared with upstream OpenCode. When upstream bumps its
own action versions we just drop the markers on merge.

.github/workflows/containers.yml

@@ -26,7 +26,7 @@ jobs:
           OWNER: ${{ github.repository_owner }}
       # kilocode_change end
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6 # kilocode_change
 
       - uses: ./.github/actions/setup-bun